Nano titanium-dioxid and cink-oxid

I read on many labels nano titanium dioxid and nano cink-oxid even in natural products. As far as I know, these physical sunscreen are the best for the skin in their "normal" forms. But as if they are nano, they might penetrate through the skin, and make problems? Is it still safe to use nano titanium-dioxid and nano cink-oxid sunscreens, and better for sensitive skin, or it is the best to avoid them?